1. Field of the Invention
The present invention relates to an electrophotographic photoreceptor. In addition, the present invention relates to an image forming apparatus and a process cartridge using the electrophotographic photoreceptor.
2. Discussion of the Background
Information processing systems using electrophotography are making remarkable progress recently. Particularly, laser printers and digital copiers, in which information is converted into digital signals and recorded by means of light, have been extremely improved in terms of printing quality and reliability. Further, technologies used in these laser printers and digital copiers are applied to laser printers and digital copiers capable of printing full-color images by merging with high-speed printing technologies. For the above reasons, photoreceptors used for such laser printers and digital copiers are required both to produce high quality images and to have high durability.
Photoreceptors using organic photoconductive materials (hereinafter “organic photoreceptors” or “photoreceptors”, unless otherwise described) are widely used for laser printers and digital copiers using electrophotography, due to their cost, manufacturability, and non-polluting properties. The organic photoreceptors are generally classified into single-layer photoreceptors and functionally-separated multilayer photoreceptors. The first practical organic photoreceptor, i.e., PVK-TNF charge-transfer complex photoreceptor was a member of the former single-layer photoreceptors.
In 1968, Hayashi and Regensburger independently invented PVK/a-Se multilayer photoreceptors. In 1977, Melts, and in 1978, Schlosser disclosed multilayer photoreceptors whose photosensitive layers, i.e., an organic pigment dispersed layer and an organic low-molecular-weight material dispersed layer, were all formed from organic material. These are known as functionally-separated multilayer photoreceptors because of having a charge generation layer (CGL) generating a charge by absorbing light and a charge transport layer (CTL) injecting and transporting the charge generated in the CGL and neutralizing charge on a surface of the photoreceptor.
However, the photosensitive layers of the organic photoreceptors are easily abraded by repeated use, and therefore the potential and photosensitivity of the photoreceptor tend to deteriorate. Further, a scratch made on the surface of the photoreceptor tends to cause background fouling and deterioration of density and quality of the resultant images. Therefore, improvement of abrasion resistance of the organic photoreceptors has been an important subject. Further, improvement of durability of the organic photoreceptors has become a more important subject recently, in accordance with speeding up of the printing speed and downsizing of an image forming apparatus attended with reduction of the diameter of the photoreceptor.
As a method of improving the abrasion resistance of the photoreceptor, methods of imparting lubricity to the photosensitive layer, hardening the photosensitive layer, including a filler therein, and using a high-molecular-weight charge transport material instead of a low-molecular-weight charge transport material are widely known. However, another problem occurs when these methods are used to prevent the abrasion of the photoreceptor. Specifically, oxidized gases, such as ozone and NOx arising due to use conditions or environment, adhere to the surface of the photosensitive layer and decrease the surface resistance thereof, resulting in a problem such as blurring of the resultant images. So far, such a problem has been avoided to some extent because the substances causing the blurred images are gradually scraped off in accordance with the abrasion of the photosensitive layer.
However, in order to comply with the above-described recent demand for higher sensitivity and durability of the photoreceptor, a new technique has been needed. In order to decrease the influence of substances causing the blurred images, there is a method of equipping the photoreceptor with a heater, which is a large drawback for downsizing the apparatus and decreasing the electric power consumption. In addition, a method of including an additive such as an antioxidant in the photosensitive layer is effective. However, the additive typically does not have photoconductivity, and therefore including much amount thereof in the photosensitive layer causes problems such as deterioration of sensitivity and increase of residual potential of the resultant photoreceptor.
As mentioned above, the photoreceptor tends not to be abraded by being given high abrasion resistance or properly process-designing around thereof. However, such photoreceptors inevitably produce blurred and low-resolution images, and it is difficult to achieve both high durability and high quality of the resultant images. The problem lies in a contradictory fact that high surface resistance of the photosensitive layer is preferable to prevent the blurred images and low surface resistance thereof is preferable to prevent the increase of residual potential.
Most practical photoreceptors are functionally-separated multilayer photoreceptors in which a charge generation layer and a charge transport layer are overlaid on a conductive substrate. The charge transport layer typically includes a hole transport material as a charge transport material. Such a photoreceptor is typically negatively chargeable.
Most copiers and printers employ corona discharge, which is a highly reliable charging method in electrophotography. However, as is generally known, negative corona discharge is unstable compared to positive corona discharge. Therefore, scorotron discharge, which is costly, is preferably employed, resulting in increasing cost. Negative corona discharge produces a large amount of ozone, which is a substance causing chemical damage. The ozone thus produced oxidizes and deteriorates a binder resin and a charge transport material. Further, ionic substances such as a nitrogen oxide ion, a sulfur oxide ion, and ammonium ion are also produced by negative corona discharge, and accumulate on the surface of the photoreceptor. Consequently, the resultant images have low image quality. In order to prevent the ozone from discharging out of the apparatus, most copiers and printers employing a negative charging method are equipped with an ozone filter, resulting in cost rise. Furthermore, the large amount of ozone thus produced causes environmental pollution.
To overcome the above-described drawbacks of negatively chargeable photoreceptors, positively chargeable photoreceptors have been proposed. A positively chargeable photoreceptor produces less ozone and nitrogen oxide ion. Moreover, a combination of a typical two-component developer with a positively chargeable photoreceptor can stably provide high quality images regardless of environmental conditions.
In a positively chargeable single-layer photoreceptor or a positively chargeable reverse multilayer photoreceptor, in which a charge generation layer is located on the outermost surface of the photoreceptor, a charge generation material is exposed at or located near the surface of the photoreceptor. Such photoreceptors have a drawback that properties thereof largely vary due to environmental gases such as exhausts from kerosene fan heaters or cars.
On the other hand, high-speed copying processes prefer negatively chargeable photoreceptors to positively chargeable photoreceptors. This is because most organic materials applicable to the practical high-speed copying processes have only hole transportability. In other words, only hole transport materials have been put to practical use. A normal multilayer photoreceptor, in which a charge transport layer including the hole transport material is located on the outermost surface of the photoreceptor, is typically negatively chargeable in principle.
If a photoreceptor is both positively and negatively chargeable, the photoreceptor may be widely applied to various fields, resulting in reduction of species of photoreceptors. Accordingly, cost may be reduced and high-speed printing may be advantageously achieved.
Japanese Patent No. (hereinafter JP) 2732697 discloses a bichargeable electrophotographic photoreceptor capable of being charged to both negative and positive polarities. The photoreceptor includes a diphenoquinone derivative as an electron transport material, but the diphenoquinone derivative has relatively low charge transportability. Therefore, photoconductive properties of the photoreceptor are not satisfactory enough in consideration of increasing printing speed and downsizing of copiers and printers. In addition, the photoreceptor may cause blurred images by repeated use.
Unexamined Japanese Application Publication No. (hereinafter JP-A) 2000-231204 discloses an electrophotographic photoreceptor including an aromatic compound having a dialkylamino group as an acid scavenger. The use of the aromatic compound provides high image quality even after the photoreceptor is repeatedly used. However, the aromatic compound has low charge transportability, and therefore the photoreceptor may not respond to the demands for high-sensitivity and high-speed printing. The added amount of the aromatic compound may be also limited.
JP-A 60-196768 and JP 2884353 have disclosed electrophotographic photoreceptors including a stilbene compound having a dialkylamino group. A reference entitled “The Effects of Nitrogen Oxide on the Resolution of Organic Photoconductors, A. Itami et al., Konica Technical Report Vol. 13 (2000)” describes that these stilbene compounds having a dialkylamino group have resistance to oxidized gases which cause image blurring.
Since the dialkylamino group, which is a substituent having a strong mesomerism effect (i.e., +M effect), is included in a resonance site of a triarylamine group structure, which is an electron transport site, the stilbene compound has an extremely small ionization potential. If such a stilbene compound is solely included in a photosensitive layer as a hole transport material, the photosensitive layer may potentially have poor charge retention ability or the charge retention ability may gradually deteriorate by repeated use. Therefore, a fatal problem lies in the fact that the stilbene compounds are difficult to be put into practical use. Even if the stilbene compound is used in combination with other charge transport material, the stilbene compound may become a hole-trapping site for moving charges because of having an extremely small ionization potential. Consequently, the resultant photoreceptor may have extremely low sensitivity and large residual potential.
JP-A 2004-258253 discloses an electrophotographic photoreceptor including a stilbene compound and a specific diamine compound in combination. The photoreceptor has an improved resistance to oxidized gases even after repeated use without decreasing sensitivity. However, high-speed printing and downsizing of the apparatus along with reducing the diameter of the photoreceptor have not realized with the photoreceptor.
German Patent No. 1230031 discloses a naphthalenetetracarboxylic acid diimide derivative used as an acceptor for formation of a charge-transfer complex. International Patent Application Publication No. 2002040479 also discloses a naphthalenetetracarboxylic acid diimide derivative pharmaceutically used as an anti-helicobacterial agent. However, use of these naphthalenetetracarboxylic acid diimide derivatives for electrophotographic photoreceptors is not mentioned at all.